Room Temperature Terahertz Electroabsorption Modulation by Excitons in Monolayer Transition Metal Dichalcogenides

The interaction between off-resonant laser pulses and excitons in monolayer transition metal dichalcogenides is attracting increasing interest as a route for the valley-selective coherent control of the exciton properties. Here, we extend the classification of the known off-resonant phenomena by unveiling the impact of a strong THz field on the excitonic resonances of monolayer MoS$_2$. We observe that the THz pump pulse causes a selective modification of the coherence lifetime of the excitons, while keeping their oscillator strength and peak energy unchanged. We rationalize these results theoretically by invoking a hitherto unobserved manifestation of the Franz-Keldysh effect on an exciton resonance. As the modulation depth of the optical absorption reaches values as large as 0.05 dB/nm at room temperature, our findings open the way to the use of semiconducting transition metal dichalcogenides as compact and efficient platforms for high-speed electroabsorption devices.Comment: 40 pages, 11 figure

Millimeter-scale exfoliation of hBN with tunable flake thickness

As a two-dimensional (2D) dielectric material, hexagonal boron nitride (hBN) is in high demand for applications in photonics, nonlinear optics, and nanoelectronics. Unfortunately, the high-throughput preparation of macroscopic-scale, high-quality hBN flakes with controlled thickness is an ongoing challenge, limiting device fabrication and technological integration. Here, we present a metal thin-film exfoliation method to prepare hBN flakes with millimeter-scale dimension, near-unity yields, and tunable flake thickness distribution from 1-7 layers, a substantial improvement over scotch tape exfoliation. The single crystallinity and high quality of the exfoliated hBN are demonstrated with optical microscopy, atomic force microscopy, Raman spectroscopy, and second harmonic generation. We further explore a possible mechanism for the effectiveness and selectivity based on thin-film residual stress measurements, density functional theory calculations, and transmission electron microscopy imaging of the deposited metal films. We find that the magnitude of the residual tensile stress induced by thin film deposition plays a key role in determining exfoliated flake thickness in a manner which closely resembles 3D semiconductor spalling. Lastly, we demonstrate that our exfoliated, large-area hBN flakes can be readily incorporated as encapsulating layers for other 2D monolayers. Altogether, this method brings us one step closer to the high throughput, mass production of hBN-based 2D photonic, optoelectronic, and quantum devices.Comment: 21 pages, 5 figures, work completed at Stanford Universit

Terahertz-driven irreversible topological phase transition in two-dimensional MoTe2_{2}

Recent discoveries of broad classes of quantum materials have spurred fundamental study of what quantum phases can be reached and stabilized, and have suggested intriguing practical applications based on control over transitions between quantum phases with different electrical, magnetic, and$/$or optical properties. Tabletop generation of strong terahertz (THz) light fields has set the stage for dramatic advances in our ability to drive quantum materials into novel states that do not exist as equilibrium phases. However, THz-driven irreversible phase transitions are still unexplored. Large and doping-tunable energy barriers between multiple phases in two-dimensional transition metal dichalcogenides (2D TMDs) provide a testbed for THz polymorph engineering. Here we report experimental demonstration of an irreversible phase transition in 2D MoTe$_{2}$ from a semiconducting hexagonal phase (2H) to a predicted topological insulator distorted octahedral ($1T^{'}$) phase induced by field-enhanced terahertz pulses. This is achieved by THz field-induced carrier liberation and multiplication processes that result in a transient high carrier density that favors the $1T^{'}$ phase. Single-shot time-resolved second harmonic generation (SHG) measurements following THz excitation reveal that the transition out of the 2H phase occurs within 10 ns. This observation opens up new possibilities of THz-based phase patterning and has implications for ultrafast THz control over quantum phases in two-dimensional materials

Strong-field Phenomena in Low-dimensional Materials at Terahertz Frequencies

The advent of terahertz(THz)-frequency laser pulses carrying a substantial fraction of their energy in a single field oscillation cycle has opened a new era in the experimental investigation of strong light-matter interactions in solids, motivated by the quest for the ultimate frontiers of all-optical controls. Exploring ways to approach those frontiers requires insight into the underlying strong-field physics. Meantime, the development of low-dimensional materials with a reduction in at least one dimension has been shown to reveal novel properties beyond those encountered in bulk forms, including the emergence of multi-phase landscapes, collective quantum effects, and topological orders. This dissertation explores strong-field phenomena in low-dimensional materials driven by strong-field pulsed excitation at THz frequencies. I first introduce the generation and detection of high-field THz and mid-infrared (MIR) pulses. The perplexing strong-field responses of low-dimensional materials urge the implementation of multimodal probe schemes and advanced theoretical frameworks. So I describe three classes of spectroscopic methods to disentangle intricate couplings and complex behaviors under THz-frequency electromagnetic irradiation. Then I elaborate on strong-field theories of non-periodic and periodic systems under oscillating fields. We have investigated two-dimensional transition metal dichalcogenides (2D TMDs) and zero-dimensional quantum dots (QDs) under electromagnetic excitation at THz frequencies. For 2D TMDs, we have explored a hitherto unobserved Franz-Keldysh effect on exciton resonance in monolayer MoS2 under THz fields. We have demonstrated a metastable topological phase transition in 2D MoTe2, driven by THz-liberated carriers assisted with coherent phonon excitations. Further single-shot measurements reveal evidence of an intermediate phase. For QDs, we have demonstrated THz-driven reemergence of quenched photoluminescence in QDs on gold by suppressing the trion-mediated Auger recombination. By effectively engineering the charge transfer between luminophore systems, we have developed a record-sensitive THz detector and polarimeter via THz-to-visible upconversion. We have investigated crossover between the quantum-mechanical and classical description of light in the QD up-conversion spanning visible, near-infrared, MIR, and THz regimes. With the above knowledge, we have demonstrated an all-optical control of fluorescence blinking in single QDs with MIR pulses by removing excess charges, thereby significantly reducing photoluminescence flicker and achieving near-unity quantum yield even at high excitation flux.Ph.D

Nonlinear rotational spectroscopy reveals many-body interactions in water molecules

Significance Since water vapor exists everywhere around us and is crucial to life, the stable complexes that water molecules form with each other and with various environmental constituents have been studied extensively. Transient, metastable complexes are more elusive. A recently developed method, two-dimensional rotational spectroscopy, directly measures correlations between the rotational transitions in a conventional spectrum. Measurements of water vapor showed that rotations of one water molecule can change the rotational frequencies of another. Distinct spectral peaks provide direct experimental signatures of previously unseen complexes between the water molecules involved. The sensitivity of the method to intermolecular interactions has directly identified metastable cooperative behavior in one of the most extensively studied molecular species and promises new insights about many others.</jats:p

All-optical fluorescence blinking control in quantum dots with ultrafast mid-infrared pulses

Photoluminescence intermittency is a ubiquitous phenomenon, reducing the temporal emission intensity stability of single colloidal quantum dots (QDs) and the emission quantum yield of their ensembles. Despite efforts to achieve blinking reduction by chemical engineering of the QD architecture and its environment, blinking still poses barriers to the application of QDs, particularly in single-particle tracking in biology or in single-photon sources. Here, we demonstrate a deterministic all-optical suppression of QD blinking using a compound technique of visible and mid-infrared excitation. We show that moderate-field ultrafast mid-infrared pulses (5.5 μm, 150 fs) can switch the emission from a charged, low quantum yield grey trion state to the bright exciton state in CdSe/CdS core-shell QDs, resulting in a significant reduction of the QD intensity flicker. Quantum-tunnelling simulations suggest that the mid-infrared fields remove the excess charge from trions with reduced emission quantum yield to restore higher brightness exciton emission. Our approach can be integrated with existing single-particle tracking or super-resolution microscopy techniques without any modification to the sample and translates to other emitters presenting charging-induced photoluminescence intermittencies, such as single-photon emissive defects in diamond and two-dimensional materials

Giant room-temperature nonlinearities in a monolayer Janus topological semiconductor

Abstract Nonlinear optical materials possess wide applications, ranging from terahertz and mid-infrared detection to energy harvesting. Recently, the correlations between nonlinear optical responses and certain topological properties, such as the Berry curvature and the quantum metric tensor, have attracted considerable interest. Here, we report giant room-temperature nonlinearities in non-centrosymmetric two-dimensional topological materials—the Janus transition metal dichalcogenides in the 1 T’ phase, synthesized by an advanced atomic-layer substitution method. High harmonic generation, terahertz emission spectroscopy, and second harmonic generation measurements consistently show orders-of-the-magnitude enhancement in terahertz-frequency nonlinearities in 1 T’ MoSSe (e.g., > 50 times higher than 2H MoS2 for 18th order harmonic generation; > 20 times higher than 2H MoS2 for terahertz emission). We link this giant nonlinear optical response to topological band mixing and strong inversion symmetry breaking due to the Janus structure. Our work defines general protocols for designing materials with large nonlinearities and heralds the applications of topological materials in optoelectronics down to the monolayer limit

Terahertz Field-Induced Reemergence of Quenched Photoluminescence in Quantum Dots

The continuous and concerted development of colloidal quantum dot light-emitting diodes over the past two decades has established them as a bedrock technology for the next generation of displays. However, a fundamental issue that limits the performance of these devices is the quenching of photoluminescence due to excess charges from conductive charge transport layers. Although device designs have leveraged various workarounds, doing so often comes at the cost of limiting efficient charge injection. Here we demonstrate that high-field terahertz (THz) pulses can dramatically brighten quenched QDs on metallic surfaces, an effect that persists for minutes after THz irradiation. This phenomenon is attributed to the ability of the THz field to remove excess charges, thereby reducing trion and nonradiative Auger recombination. Our findings show that THz technologies can be used to suppress and control such undesired nonradiative decay, potentially in a variety of luminescent materials for future device applications